1. Field of the Invention
The present invention relates to a method of manufacturing an optical element, including a step of machining and washing a substrate made of CaF2 single crystalline or the like, an exposure apparatus using the optical element, and the optical element washing machine. In particular, the present invention relates to a method of manufacturing a fluoric lens which is used in the case where a high function is required, which is cut from a CaF2 single crystalline substrate and machined, and has a large aperture and high profile irregularity.
2. Related Background Art
Conventionally, an optical element such as a lens made of a fluoride series crystal material such as CaF2 or MgF2 has preferable transmittance in an optical characteristic over an extremely wide wavelength band and low dispersiveness. Thus, this optical element has been used as a high precision lens of which a high function is required, such as a high grade camera lens or a television camera lens.
Also, the optical element such as the lens made of the fluoride series crystal material such as CaF2 or MgF2 has high transmittance with respect to ultraviolet radiation by an excimer laser. Thus, use of such an optical element is started for various optical elements of an illuminating optical system and those of a projection optical system in a projection exposure apparatus using ultraviolet radiation with a wavelength of about 248 nm (KrF excimer laser), ultraviolet radiation with a wavelength of 193 nm (ArF excimer laser), or ultraviolet radiation with a wavelength of 157 nm (F2 excimer laser).
In manufacturing such kind of lens, generally, a wash solution is poured into a wash bath and then a lens (substrate) which has undergone surface machining is immersed therein. In this state, the lens is washed by a ultrasonic cleaning method or the like to remove a contamination such as an organic material (such as oil) or an inorganic particle (such as abrasive) on the surface of the lens. Thus, the reduction in the transmittance of the lens due to the contamination is prevented.
However, it has been found that even if such washing is made, it is still impossible to obtain a lens with predetermined transmittance. One of the reasons is the existence of a deterioration layer on the surface of the substrate, which is produced during the lens machining.